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<div class="titlepage"><div><div><h4 class="title">
<a name="boost_optional.tutorial.design_overview.the_interface"></a><a class="link" href="the_interface.html" title="The Interface">The
        Interface</a>
</h4></div></div></div>
<p>
          Since the purpose of optional is to allow us to use objects with a formal
          uninitialized additional state, the interface could try to follow the interface
          of the underlying <code class="computeroutput"><span class="identifier">T</span></code> type
          as much as possible. In order to choose the proper degree of adoption of
          the native <code class="computeroutput"><span class="identifier">T</span></code> interface,
          the following must be noted: Even if all the operations supported by an
          instance of type <code class="computeroutput"><span class="identifier">T</span></code> are
          defined for the entire range of values for such a type, an <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
          extends such a set of values with a new value for which most (otherwise
          valid) operations are not defined in terms of <code class="computeroutput"><span class="identifier">T</span></code>.
        </p>
<p>
          Furthermore, since <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> itself is merely a <code class="computeroutput"><span class="identifier">T</span></code>
          wrapper (modeling a <code class="computeroutput"><span class="identifier">T</span></code> supertype),
          any attempt to define such operations upon uninitialized optionals will
          be totally artificial w.r.t. <code class="computeroutput"><span class="identifier">T</span></code>.
        </p>
<p>
          This library chooses an interface which follows from <code class="computeroutput"><span class="identifier">T</span></code>'s
          interface only for those operations which are well defined (w.r.t the type
          <code class="computeroutput"><span class="identifier">T</span></code>) even if any of the operands
          are uninitialized. These operations include: construction, copy-construction,
          assignment, swap and relational operations.
        </p>
<p>
          For the value access operations, which are undefined (w.r.t the type <code class="computeroutput"><span class="identifier">T</span></code>) when the operand is uninitialized,
          a different interface is chosen (which will be explained next).
        </p>
<p>
          Also, the presence of the possibly uninitialized state requires additional
          operations not provided by <code class="computeroutput"><span class="identifier">T</span></code>
          itself which are supported by a special interface.
        </p>
<h6>
<a name="boost_optional.tutorial.design_overview.the_interface.h0"></a>
          <span class="phrase"><a name="boost_optional.tutorial.design_overview.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_uninitialized_optional_objects__the_operators___and___gt_"></a></span><a class="link" href="the_interface.html#boost_optional.tutorial.design_overview.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_uninitialized_optional_objects__the_operators___and___gt_">Lexically-hinted
          Value Access in the presence of possibly uninitialized optional objects:
          The operators * and -&gt;</a>
        </h6>
<p>
          A relevant feature of a pointer is that it can have a <span class="bold"><strong>null
          pointer value</strong></span>. This is a <span class="emphasis"><em>special</em></span> value
          which is used to indicate that the pointer is not referring to any object
          at all. In other words, null pointer values convey the notion of nonexistent
          objects.
        </p>
<p>
          This meaning of the null pointer value allowed pointers to became a <span class="emphasis"><em>de
          facto</em></span> standard for handling optional objects because all you
          have to do to refer to a value which you don't really have is to use a
          null pointer value of the appropriate type. Pointers have been used for
          decades—from the days of C APIs to modern C++ libraries—to <span class="emphasis"><em>refer</em></span>
          to optional (that is, possibly nonexistent) objects; particularly as optional
          arguments to a function, but also quite often as optional data members.
        </p>
<p>
          The possible presence of a null pointer value makes the operations that
          access the pointee's value possibly undefined, therefore, expressions which
          use dereference and access operators, such as: <code class="computeroutput"><span class="special">(</span>
          <span class="special">*</span><span class="identifier">p</span>
          <span class="special">=</span> <span class="number">2</span> <span class="special">)</span></code> and <code class="computeroutput"><span class="special">(</span>
          <span class="identifier">p</span><span class="special">-&gt;</span><span class="identifier">foo</span><span class="special">()</span> <span class="special">)</span></code>, implicitly convey the notion of optionality,
          and this information is tied to the <span class="emphasis"><em>syntax</em></span> of the
          expressions. That is, the presence of operators <code class="computeroutput"><span class="special">*</span></code>
          and <code class="computeroutput"><span class="special">-&gt;</span></code> tell by themselves
          —without any additional context— that the expression will be undefined
          unless the implied pointee actually exist.
        </p>
<p>
          Such a <span class="emphasis"><em>de facto</em></span> idiom for referring to optional objects
          can be formalized in the form of a concept: the <a href="../../../../../../utility/OptionalPointee.html" target="_top"><code class="computeroutput"><span class="identifier">OptionalPointee</span></code></a> concept. This
          concept captures the syntactic usage of operators <code class="computeroutput"><span class="special">*</span></code>,
          <code class="computeroutput"><span class="special">-&gt;</span></code> and contextual conversion
          to <code class="computeroutput"><span class="keyword">bool</span></code> to convey the notion
          of optionality.
        </p>
<p>
          However, pointers are good to <span class="underline">refer</span>
          to optional objects, but not particularly good to handle the optional objects
          in all other respects, such as initializing or moving/copying them. The
          problem resides in the shallow-copy of pointer semantics: if you need to
          effectively move or copy the object, pointers alone are not enough. The
          problem is that copies of pointers do not imply copies of pointees. For
          example, as was discussed in the motivation, pointers alone cannot be used
          to return optional objects from a function because the object must move
          outside from the function and into the caller's context.
        </p>
<p>
          A solution to the shallow-copy problem that is often used is to resort
          to dynamic allocation and use a smart pointer to automatically handle the
          details of this. For example, if a function is to optionally return an
          object <code class="computeroutput"><span class="identifier">X</span></code>, it can use <code class="computeroutput"><span class="identifier">shared_ptr</span><span class="special">&lt;</span><span class="identifier">X</span><span class="special">&gt;</span></code>
          as the return value. However, this requires dynamic allocation of <code class="computeroutput"><span class="identifier">X</span></code>. If <code class="computeroutput"><span class="identifier">X</span></code>
          is a built-in or small POD, this technique is very poor in terms of required
          resources. Optional objects are essentially values so it is very convenient
          to be able to use automatic storage and deep-copy semantics to manipulate
          optional values just as we do with ordinary values. Pointers do not have
          this semantics, so are inappropriate for the initialization and transport
          of optional values, yet are quite convenient for handling the access to
          the possible undefined value because of the idiomatic aid present in the
          <a href="../../../../../../utility/OptionalPointee.html" target="_top"><code class="computeroutput"><span class="identifier">OptionalPointee</span></code></a>
          concept incarnated by pointers.
        </p>
<h6>
<a name="boost_optional.tutorial.design_overview.the_interface.h1"></a>
          <span class="phrase"><a name="boost_optional.tutorial.design_overview.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee"></a></span><a class="link" href="the_interface.html#boost_optional.tutorial.design_overview.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee">Optional&lt;T&gt;
          as a model of OptionalPointee</a>
        </h6>
<p>
          For value access operations <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code> uses operators <code class="computeroutput"><span class="special">*</span></code>
          and <code class="computeroutput"><span class="special">-&gt;</span></code> to lexically warn
          about the possibly uninitialized state appealing to the familiar pointer
          semantics w.r.t. to null pointers.
        </p>
<div class="caution"><table border="0" summary="Caution">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../../../../../doc/src/images/caution.png"></td>
<th align="left">Caution</th>
</tr>
<tr><td align="left" valign="top"><p>
            However, it is particularly important to note that <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code> objects are not pointers. <span class="underline"><code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code> is not, and does not model, a
            pointer</span>.
          </p></td></tr>
</table></div>
<p>
          For instance, <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code> does not have shallow-copy so does
          not alias: two different optionals never refer to the <span class="emphasis"><em>same</em></span>
          value unless <code class="computeroutput"><span class="identifier">T</span></code> itself is
          a reference (but may have <span class="emphasis"><em>equivalent</em></span> values). The
          difference between an <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> and a pointer must be kept in mind,
          particularly because the semantics of relational operators are different:
          since <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
          is a value-wrapper, relational operators are deep: they compare optional
          values; but relational operators for pointers are shallow: they do not
          compare pointee values. As a result, you might be able to replace <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
          by <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span></code>
          on some situations but not always. Specifically, on generic code written
          for both, you cannot use relational operators directly, and must use the
          template functions <a href="../../../../../../utility/OptionalPointee.html#equal" target="_top"><code class="computeroutput"><span class="identifier">equal_pointees</span><span class="special">()</span></code></a>
          and <a href="../../../../../../utility/OptionalPointee.html#less" target="_top"><code class="computeroutput"><span class="identifier">less_pointees</span><span class="special">()</span></code></a>
          instead.
        </p>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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